The concept "adjustment of the braking value", "disappearance of the deviation" and terms of similar meanings must always be understood in this text to be within the limits of practically unavoidable tolerances.
The areas of application for the invention are braking systems which, in conjunction with an object to be braked, form a closed loop control circuit in which the braking value is the adjusted value and the brake application energy is the actuating medium. A desired braking value is demanded from such a braking system by feeding it with a desired-braking value demand. The braking system is prompted to meet the desired braking value demand by automatically making the appropriate adjustment and variation to the brake application energy. A process of this type is known from DE 35 02 825.
The at least one brake of a braking system produces a braking force when brake application energy is supplied. The braking value produced by the braking system is a parameter which depends on the brake's brake force. The braking value depends exclusively on the brake force, when the force represents the braking value itself. However, the braking value, in addition to the brake force of the brake, is generally determined by parameters of the braked object. In automotive engineering, as disclosed in DE 35 02 825 A1, the brake force between the wheel and the road, as well as the vehicle deceleration, are generally used as the braking value. Here dimensions of the vehicle or of vehicle parts or the weight of the vehicle contribute in a known manner in determining the braking value.
Generally, the following formula applies to the dependency between the brake application energy and the braking value: EQU B=F*M*C*Z (I)
In this formula,
B is the braking value; PA1 Z is the brake application energy; PA1 C is a coefficient defining the brake characteristics inclusive of possible transmission ratios; PA1 F is a coefficient defining the weight of the braked object; and PA1 M is a coefficient defining brake-relevant dimensions of the braked object.
Depending on the type of braking value selected, one or several of the coefficients C, F and M may be constant or variable. If, for instance, the vehicle deceleration is taken as the braking value, then F is a variable. On the other hand, one or more of the coefficients C, F and M can be set as insignificant and equal to 1, depending on the type of braking value selected. This applies in automotive engineering, for example, to F if the brake force between the wheel and the road serves as braking value.
The schematic brake force brake application energy diagram of FIG. 1 illustrates that the brake has characteristic curves representing the produced brake force (K) as a function of the brake application energy (Z) for brake activation and for brake release. These curves take different courses because of the brake's hysteresis. During brake actuation, i.e. when the brake application energy (Z) is increasing or remains unchanged after having risen, the brake force (K) follows characteristic curve (X) starting from a brake application energy value ZA. The brake application energy value ZA is the value required to overcome the response resistances. When the maximum value ZOU of the brake application energy (Z) has been reached, the brake produces the greatest possible brake force KM. The maximum value ZOU is determined by the energy supply of the brake system. When the brake is released, i.e., when the brake application energy (Z) drops or remains unchanged after having dropped, the brake continues to produce the brake force or pressure KM until the brake application energy (Z) has dropped to a value ZOT. Only in the case of a further drop in the brake application energy (Z) does the brake force (K) also drop. This drop in brake force (K) is represented by the characteristic curve (Y). The behavior of the brake with a lower brake application energy (Z) and corresponding brake force (K) is analogous. Starting, for instance, from an intersecting point with the coordinates, such as in brake application energy Z1 and brake force K1, the drop in the brake force only begins after a drop in the brake application energy to a value Z2. The horizontal difference in brake application energy (Z) between the characteristic curves (X, Y) associated with each brake force (K) is the applicable hysteresis. The area between the characteristic curves (X, Y) is the hysteresis field. Because of the response resistances, as the brake is released, the brake force (K) becomes equal to zero with only a residual brake application energy ZR.
This property of the brake effects the braking system because the braking value (B) also follows different characteristic curves for brake actuation and brake release as functions of the brake application energy (Z). These curves are illustrated in FIG. 2 as corresponding characteristic curves (U) and (T). In this figure, the difference in brake application energy (Z) between the characteristic curves (U and T) and the area between the characteristic curves (U, T) represent the hysteresis (Hy) and the hysteresis field, respectively, of the braking system. FIG. 2 is analogous to FIG. 1, in that the brake application energy values, ZA, ZR, ZOU and ZOT, determine the starting and end points of the characteristic curves (U, T). If the braking system is, for example, actuated up to a point with the coordinates, such as at the desired braking value BS and the associated brake application energy ZS and if the brake application energy (Z) is decreased, then the difference in brake application energy ZS between the characteristic lines (U, T) assigned to the desired braking value BS, i.e., the hysteresis HyS, must be overcome before the braking system responds and decreases the braking value (B).
The above-indicated formula (I) only applies in the case of brake actuation because of the hysteresis. Brake actuation occurs when the brake application energy (Z) is rising or when the brake application energy (Z) remains unchanged after having risen.
Based on the braking value (B) reached in actuating the brake, according to formula (I), the following general formula applies to the hysteresis (Hy): EQU HY=ZA-ZR+(ZOU-ZOT-ZA+ZR)*((B/F*M*C)-ZA)/(ZOU-ZA) (II)
The presence of a hysteresis is disadvantageous. This also applies when a deviation of the existing actual braking value from the demanded desired braking value occurs during a brake actuation. In other words, when the actual braking value becomes too great.
In order to eliminate this deviation, the conventional process provides two alternatives. In the first alternative, the known process decreases the brake application energy in steps until the hysteresis has been overcome and the deviation has disappeared. In the second alternative, the known process decreases the brake application energy with a comparatively wide leap which leads to falling short of the desired braking value. Then the brake application energy is increased in steps until the desired braking value has again been reached. These alternatives result in a discontinuous course of the braking value which is sensed by the operator as an unsatisfactory degree of control of the braking system.